Such metals, sometimes referred to as “valve” metals include for example magnesium, aluminium, titanium, tantalum, zirconium, chromium, vanadium, cobalt, hafnium, molybdenum and any of their alloys. The resultant oxide layer provides some degree of corrosion protection because it constitutes a physical barrier between the metal and the corrosive environment. An alternative route for the protection of these metals is chemical passivation, whereby a thin film is formed on the metal surface by chemical reaction. Such films can provide continued, active, corrosion protection by reacting preferentially with any freshly exposed metal that might arise through mechanical action or corrosion.
Plasma electrolytic oxidation technology is a development of more conventional anodising technology, where different electrolytes are used and higher potentials and current densities (typically 10 to 200 mAcm−2 as compared to 1-2 mAcm−2 for more conventional anodising) are applied in order to achieve microscopic plasma discharges which modify the growing oxide film. It is sometimes also referred to as micro-arc oxidation, spark anodising or discharge anodising and other combinations of these terms. The technology has been developed for the surface protection of a wide range of metals, known as “valve” metals. These are metals which exhibit electrical rectifying behaviour in the electrolytic cell: under a given applied current, they will sustain a higher potential when anodically charged than when cathodically charged. Such metals include aluminium, magnesium, titanium, zirconium, hafnium, chromium, cobalt, molybdenum, vanadium and tantalum for example and their alloys.
Known processes for plasma electrolytic oxidation include: U.S. Pat. No. 3,293,158 (Anodic spark reaction processes and articles—McNeill et al.), U.S. Pat. No. 5,792,335 (Anodization of magnesium and magnesium based alloys—Barton et al.), U.S. Pat. No. 6,365,028 (Method for producing hard protection coatings on articles made of aluminum alloys—Shatrov), and U.S. Pat. No. 6,896,785 (Process and device for forming ceramic coatings on metals and alloys, and coatings produced by this process—Shatrov et al.).
There are many patented and commercial variants of this process, the main variants being the applied electrical regime and electrolyte. Electrical regimes include direct current, pulsed direct current and a wide range of pulsed or alternating current regimes. Electrolyte systems are also very varied but the most commercially successful systems are aqueous, alkaline solutions. Several viable processes are described within the prior art of U.S. Pat. No. 6,365,028. For example, U.S. Pat. No. 5,616,229 specifies a modified sine wave form at industrial (50-60 Hz) frequency from a source of at least 700V, and electrolytes consisting of KOH (at 0.5 g/l) with up to 11 g/l of sodium tetrasilicate. This is one of the simpler electrolyte systems and is not stable. U.S. Pat. No. 6,365,028 employs a more stable electrolyte consisting of an aqueous solution of an alkaline metal hydroxide at 1-5 g/l, an alkali metal silicate at 2-15 g/l, an alkaline metal pyrophosphate at 2-20 g/l and peroxide compounds at 2-7 g/l.
The benefits of plasma electrolytic oxidation of a surface include both mechanical protection and corrosion protection. The mechanical protection is due to the formation of a hard, well-adhered layer of ceramic. The oxide layers tend to be significantly harder than more conventional hard anodised layers because the plasma discharge processes convert amorphous oxides into harder crystalline forms such as the alpha phase of alumina.
Because plasma electrolytic oxide films constitute a corrosion resistant, barrier layer of oxide on the surface of a metal, they present a protective barrier which isolates that metal from any corrosive environments. As such, they can extend the life of metal components in environments which would otherwise result in rapid corrosion and degradation of the metal surfaces.
It is known, for example from CA2540340, to improve a coating process by pre-treating the surface of an aluminium product by first forming a thin, dense and non-porous alumina barrier layer (dielectric layer) through anodizing, electrolytic oxidation, chemical oxidation, physical vapour deposition and/or chemical vapour deposition. A modification substance layer comprising metal oxide, carbide, boride, nitride, silicide and/or solid lubricant or composites is formed on top of the alumina barrier layer by the same techniques, or by powder spray techniques. Once the surface has been prepared in this way, micro-arc oxidation is commenced, the resulting oxide coating being said to be improved over an oxide coating formed on an untreated aluminium surface. The modification substance layer is said to promote micro-arc fusing, to promote oxide growth, to provide permanent lubrication or hardening and to improve smoothness/hardness so as to reduce the need for subsequent machining. There is no mention whatsoever of corrosion inhibition or chemical passivation.
The main weakness of plasma electrolytic oxide films in terms of corrosion protection is that they are mere physical barriers, and as soon as they are physically breached, they leave an area of the substrate exposed to the environment and vulnerable to corrosive attack. For this reason, sealers and top-coats are often applied to provide a less permeable, thicker and/or tougher barrier. These exploit the fine and varied surface-connected pore structure of the micro arc oxide films [see e.g. “Porosity in plasma electrolytic oxide coatings”, J. A. Curran and T. W. Clyne, Acta Materialia 54 (2006) pp 1985-1993] which enables impregnation to form a composite layer or intimate bonding of a top-coat.
Typical sealer systems used in conjunction with plasma electrolytic oxide coatings include a wide range of polymers including but not limited to fluoropolymers (e.g. DE4124730 Intercalation of fluorinated polymer particles—into microporous oxide surfaces of aluminium, magnesium and aluminium magnesium alloy objects for homogeneous coating of polymers—AHC Oberflachentechnik), acrylic, epoxy, polyester, polysiloxanes and PVDF. These are typically applied in the form of electrostatically sprayed powder coats, by electrophoretic deposition (e.g. WO 99/02759 —Sealing procedures for metal and/or anodised metal substrates—MacCulloch and Ross), or simply by dipping or wet spraying. “Primer” systems such as tetra methyl silane will often be used as an intermediate treatment to enhance the adhesion of polymeric top-coats. Inorganic sealing or top-coating treatments for plasma electrolytic oxide coatings include silica (which is typically applied in the form of an aqueous sodium silicate solution dip), and sol-gels. Lubricants are often applied to plasma electrolytic oxide coatings to fill pore structures while enhancing tribological performance. These include oil-based lubricants but also solid state lubricants such as graphite, boron nitride (BN), or molybdenum disulphide (MoS2), and numerous polymeric lubricants such as the previously mentioned PTFE dispersions. Even top-coats of metals such as nickel have been used in conjunction with plasma electrolytic oxide coatings (WO 01/12883 Light alloy-based composite protective multifunction coating—Shatrov et al), and these may be applied by techniques as diverse as plasma spraying, electroplating, and electroless deposition.
This is similar to the development of duplex systems with anodising, as described in U.S. Pat. No. 5,439,747 and U.S. Pat. No. 6,905,775.
WO 97/05302 discloses a post treatment for a micro-arc or plasma-electrolytic oxidation coating in which the coating is physically sealed using a silicic acid sol gel. The sol gel is used to seal porosity in the coating, and any chemical activity arising from compounds in the sol gel is confined to reaction with the oxide coating, with no regard being given to the underlying metal. While passing mention is made of the optional provision of corrosion inhibitors in the sol gel, it is clear that such corrosion inhibitors (which are not disclosed in any detail) are limited to those that can be incorporated in a silicic acid sol gel that is used for post-sealing the pores of a micro-arc oxidation or PEO coating.
U.S. 2006/0016690 discloses a micro-arc oxidation process in which additional compounds or moieties are included in the liquid electrolyte with the intention of, among other things, improving corrosion resistance. This is a “one step” process—there is no separation of chemical and physical treatment steps.
Another post-treatment for PEO coatings is known from EP1231299 to the present Applicant. This discloses the incorporation of various functional components including various transition element metals and their carbides, oxides, nitrides, borides and silicides into the pores of the PEO coating. The purpose of these components is to reduce friction and to provide resistance to wear and scratching, not to enhance corrosion protection. While resistance to wear and scratching will in itself provide some passive corrosion resistance, there is no disclosure of any mechanism for active corrosion protection or chemical passivation in the event of a breach in the oxide layer exposing the underlying metal.
None of these systems for the enhancement of micro arc oxide layers includes a chemically active agent, designed to afford continued active protection to the metal in the event of a physical breach of the oxide layer. The function of the secondary treatments in existing plasma electrolytic oxidation technology is to physically seal the pore structure, to promote the adhesion of further top-coat systems, to physically augment the protective coating in terms of thickness or mechanical robustness, or to modify physical attributes of the layer (such as its wear performance, friction coefficient, toughness, colour, reflectivity, electrical continuity etc.).
Chemical conversion or passivation is a well-developed technology for the corrosion protection of metals, in its own right. It is often also used as pre-treatment for further polymeric top-coats. The most effective system for the chemical conversion treatment of aluminium and magnesium, for example, is chromate conversion treatment. Typical examples include chromic acid/chromate treatments such as a solution of chromic acid (CrO3) and hydrofluoric acid (HF), often with an accelerator. An alternative is phosphoric acid/chromate treatment such as that disclosed in U.S. Pat. No. 2,438,877 where the conversion treatment solution is composed of chromic acid (CrO3), phosphoric acid (H3PO4) and hydrofluoric acid (HF). This solution produces a protective surface film of chromium phosphate (CrPO4.4H2O).
Chromate based conversion coatings such as those described above and that described in U.S. Pat. No. 5,451,271 have been widely adopted by industry for the corrosion protection of metals such as aluminium, magnesium and their alloys. However, the high toxicity of chromate based systems has necessitated their replacement and the development of a wide range of alternatives such as zinc phosphate based conversion coating and other systems including fluorides and zirconates and titanates for example. Zinc phosphate conversion coatings are formed by exposing a clean, active metal surface to an aqueous acidic solution containing zinc and phosphate ions. For example, the result of a mixture in water of zinc oxide and phosphoric acid results in a solution containing zinc dihydrogen phosphate(Zn(H2PO4)2). The zinc dihydrogen phosphate complexes with the metal surface to form a protective film containing zinc phosphates. There are many patented and commercial embodiments of zinc phosphating, many of which include a polyhydric polymer to quench the reactivity of the phosphating composition and aid wetting of the substrate, and other additives to aid the adhesion of further sealants or top-coat films. Examples include U.S. Pat. Nos. 5,261,973 , 5,378,292, and U.S. Pat. No. 6,117,251.
Other phosphate-based conversion processes include those disclosed in U.S. Pat. Nos. 4,264,378 and 5,520,750 (e.g. phosphate/vanadate, phosphate/tungstate or phosphate/molybdate processes), and U.S. Pat. No. 5,595,611 (a manganese phosphate conversion coating). U.S. Pat. Nos. 5,683,522 and 6,887,320 for the conversion coating of magnesium describe processes where both phosphate and fluoride ions are used in solution to form a conversion coating of magnesium phosphate (Mg3(PO4)2) and magnesium fluoride (MgF2).
In addition to chromate or phosphate based systems, passivation of metals such as aluminium or magnesium may be achieved using complex fluorides of elements such as titanium, zirconium, hafnium, silicon or boron, as exemplified by U.S. Pat. Nos. 4,298,404 and 5,584,946.
Furthermore, some chemical conversion coatings are designed to promote adhesion of a polymeric topcoat. Unfortunately, many of those designed to fulfil this function, have poor passivation performance, leading to corrosion problems if the top coat is breached. With the proposed invention, the user may use the best chemical passivation techniques, since the topcoat adhesion will be supplied by the plasma electrolytic oxide layer.
All of the chromate-free chemical conversion coatings fail to match the performance that has become expected by users of chromate based systems, as demonstrated in industry test programmes such as that reported in “Evaluation of Corrosion Protection Methods for Magnesium Alloys”, Blanchard P. J. et al., Magnesium Technology 2005, TMS (The Minerals, Metals & Materials Society) 2005.
Moreover, none of the proponents of chemical passivation identifies a synergistic combination of the two technologies. Indeed, U.S. Pat. No. 5,683,522 above explicitly describes both the shortcomings and difference of electrolytic oxidation as compared to chemical conversion, concluding that a non-electrolytic process is more suitable: “with complex shapes, as in the case of aircraft generator housings, non-uniform coatings are formed from the process of anodizing, as internal areas on the housing are either left uncoated or extremely thin, while other areas near the current application exhibit excess build-up of coating. In addition to forming non-uniform coatings, an electrolytic process does not tolerate dissimilar metals being in contact with a magnesium product during the coating step. [. . .] Inserts must be masked during the anodizing process, and when the mask is removed, an area of magnesium surrounding the insert is left uncoated.”. This is typical of the present state-of-the-art, and of the approach of those experienced in the art of corrosion and wear protection of these metals: there exist two separate protection routes, each with perceived advantages and disadvantages, and with no compatibility or synergy.
While modern plasma electrolytic oxide coatings can satisfy typical corrosion protection requirements in their own right (as shown in the Blanchard et al work), the very different nature of the corrosion protection they afford presents difficulties and prompted the development of the present invention which overcomes some of the main limitations of each of the hitherto competing technologies of electrolytic oxidation and chemical passivation.